1. Field of the Invention
The present invention relates to an optical waveguide element.
2. Description of the Related Art
Recent years have seen the remarkable development of the information society, and, in particular, the exchange of information including large volumes of data, such as moving images or the like, has increased, not only between businesses but also between individuals. For this reason, high speed communication techniques for large volumes of data have been demanded.
One of the techniques that support high speed communication for large volumes of data is an optical communication technique. Optical waveguide elements such as optical fibers, optical switching elements, optical modulators, and routers and the like are examples of elements used in optical communication.
In recent years, there has been much activity in the development of a waveguide-type optical element (i.e. an optical waveguide element) which controls guided light by an electro-optic (hereinafter, sometimes abbreviated as “EO”) effect. Various waveguide structures for the optical waveguide element have been studied, and various methods of forming a waveguide have been disclosed.
Hitherto, as a material of an optical waveguide of an optical waveguide element, inorganic materials such as lithium niobate (LiNbO3) or lead titanate zirconate having lanthanum added thereto ((Pb,La)(Zr,Ti)O3, sometimes abbreviated as “PLZT”), which exhibit prominent EO effects, have been widely used. However, when these materials are used, response speed is slow owing to the high dielectric constants of the materials, and therefore, the frequency region to which they may be applied has been limited. In addition, the manufacturing costs are high, due to, for example, a complicated manufacturing process, and the need to perform processing at high temperatures, and thus the application of inorganic materials has been limited.
In contrast, polymers have a lower dielectric constant than inorganic materials, and can significantly overcome problems of velocity mismatching with microwaves; further, the size of a waveguide is not restricted with a polymer, because film formation using a polymer is easily performed by a spin coating method or the like. In addition, since polymers are readily processed using techniques such as micro fabrication and molding processing, they have a significant advantage in that they can be manufactured into elements at an extremely low cost. As a result, polymers have attracted attention for use as an optical waveguide material.
A polymer optical waveguide element including a polymer may be produced by a method including: melting or dissolving polymer materials or polymer precursor compounds each of which forms a lower cladding layer, a waveguide layer, or an upper cladding layer; sequentially applying and curing the polymer materials or polymer precursor compounds on a substrate made of silicon or the like; and cutting or polishing an end surface of the element into a mirror plane. In addition, a waveguide is formed by combining well-known techniques such as lithography and etching. When making a device utilizing a non-linear optical effect such as an electro-optic effect, an electrode is arranged on a substrate or an upper cladding layer. A photocurable adhesive or a thermosetting adhesive is generally used for a cladding material, and a solution in which a polymer compound is dissolved in an organic solvent is generally used for a waveguide layer material.
In an optical waveguide element which controls light by using an EO effect, it is necessary to modulate or switch light input from a light source in accordance with an electric signal, and extract the modulated or switched light to the outside. For this reason, after plural optical waveguide elements are fabricated on a wafer substrate and cut into individual elements, the element is optically connected in such a manner that a fiber for light input is fixed at input and output ends of the waveguide, and, at the same time, the element is fixed to a module casing and is electrically connected, by a method such as wire bonding or flip chip mounting, to a bonding pad which is wired to a control electrode for applying an electric signal.
Hitherto, a control electrode formed on the optical waveguide element has been formed by lift-off or etching after patterning, by using a photomask, of an electrode layer formed on an upper part of an element from the viewpoint of easy processability. In this method, since a bonding pad for electrical connection is also formed on a waveguide thin film, there has been a problem in that the waveguide thin film formed with an organic material is damaged by heat or high frequency waves upon wire bonding, leading to exposure of the substrate.
A method using wire bonding, as well as other methods, are known for electrically connecting an optical waveguide element. However, conventional methods such as this have not addressed problems that accompany electrical connection of an optical waveguide element that includes an organic material. That is, even in a wire bonding device which reduces damage to a substrate with wire bonding, although conventional methods are effective in preventing destruction of an element including a material which is relatively hard and brittle, such as an inorganic optical crystal that includes LiNbO3, they are ineffective when a material which is soft, and easily affected by heat, is used, such as a polymer material. In addition, in a method of flip chip mounting, since it is necessary to melt solder at a high temperature, conventional methods cannot be used in an organic material due to a problem of heat resistance. Further, in a method using a connector, it is impossible to obtain wiring at a high density, and it is also necessary to perform shape processing of an element in order to mount a connector, and this leads to increased costs. In addition, since connection with a connector leads to increase in connection impedance, application to an element which performs high speed optical control is difficult.
As described above, despite expectations regarding the various advantages of optical waveguide elements, due to the problems associated with mounting, application for practical use has been difficult.